Whirring noise to left of dash, 2009 package VI

Hi all,
Loving my prius. I've noticed when i enter, and often after driving for a few minutes without breaks, when i then remove my foot from the accellator and either coast a bit, or use the breaks, or let off the breaks, i hear a whirring type of noise to the left - it's not grinding and i'll wager it should be there.
i'm aware of the thermos/coolant pump, i don't think that's it.
might it be the break fluid, or similar?
i THINK i always hear it after driving a bit and then depressing the break, but not sure.

also, i'll make this question a 2-fer - how do i know when i'm using regenerative braking on a gentle brake versus the brake pads, or can both be at play? i know below 7 we don't get regenerative. should we gently depress the brake for a longer period than simply coast and regenerate? just curious, thanks!

The noise is the brake accumulator pump.

To see if you're doing regen, use the Energy screen on the MFD. Yellow arrows going to the battery is regen.

There is no way to know if you're using the actual brakes. They will come into play when required, which is determined by the computer based on speed, battery condition, the force you apply to the pedal, etc.

As for regen strategy, there are whole threads on that question.. I'm not sure it's been definitively answered.

Thanks - i had a feeling it was brake related - it's not annoying, just something i was wondering about.
too bad there's not way to know instant regeneration (kind of like the fuel consumption) so then we could determine if it was more efficent to coast with regen and then apply brakes firmly, or gently regen the whole coast and stop with brakes - i think the latter versus the former - thanks again!

I was confused by that at first too.

also expect to hear the coolant pump too when you turn off the car sometimes

The brake accumulator pump runs about every third time you press the brake (roughly). It build pressure for the power brakes, since the Prius can't use a vacuum system. If it starts running all of the time you have a pressure leak. Intermittent operation is normal.

Tom